Crystal Structure of the α Subunit of Human Translation Initiation Factor 2B

Eukaryotic translation initiation factor 2B (eIF2B) is the heteropentameric guanine-nucleotide exchange factor specific for eukaryotic initiation factor 2 (6172). Under stressed conditions, guanine-nucleotide exchange is strongly inhibited by the tight binding of phosphorylated eIF2 to eIF2B. Here, we report the crystal structure of the alpha subunit of human eIF2B at 2.65 angstrom resolution. The eIF2B alpha structure consists of the N-terminal alpha-helical domain and the C-terminal Rossmann-fold-like domain. A positively charged pocket, whose entrance is about 15-17 angstrom in diameter, resides at the boundary between the two domains. A sulfate ion is located at the bottom of the pocket (about 16 angstrom in depth). The residues comprising the sulfate-ion-binding site are strictly conserved in eIF2B alpha. Since this deep, wide pocket with the sulfate-ion-binding site is not conserved in distant homologues, including 5-methylthioribose 1-phosphate isomerases, these characteristics may be distinctive of eIF2B alpha. Interestingly, the yeast eIF2B alpha missense mutations that reduce the eIF2B sensitivity to phosphorylated eIF2 are mapped on the other side of the pocket. One of the three human eIFB2B alpha missense mutations that induce the lethal brain disorder vanishing white matter or childhood ataxia with central nervous system hypomyelination is mapped inside the pocket. The beta and delta subunits of eIF2B are homologous to eIF2B alpha and may have tertiary structures similar to the present eIF2B alpha structure. The sulfate-ion-binding residues of eIF2B alpha are well conserved in eIF2B beta/delta. The abovementioned yeast and human missense mutations of eIF2B beta/delta were also mapped on the eIF2B alpha structure, which revealed that the human mutations are clustered on the same side as the pocket, while the yeast mutations reside on the opposite side. As most of the mutated residues are exposed on the surface of the eIF2B subunit structure, these exposed residues are likely to be involved in either the subunit interactions or the interaction with eIF2. (C) 2009 Elsevier Ltd. All rights reserved.